1. Field of the Invention
The present invention relates generally to control systems for industrial processes, and more particularly, to pneumatic valve positioning systems and electrical circuits thereof with steady state gas bleed prevention.
2. Description of the Related Art
Many industrial processes involve the movement of fluid such as gas, steam, water, and chemical compounds. The flow of the fluid is regulated by a control valve that has a passageway that is selectively opened and closed with a movable obstruction or valve element connected to a stem. An actuator, in turn, is connected to the stem, and provides the motive force to open and close the valve element. Pneumatic, hydraulic, electrical, or mechanical energy is converted by the actuator to linear or rotational motion, depending on the configuration of the control valve.
A conventional pneumatic actuator is comprised of a piston sealed within a cylinder, and the piston includes a connecting rod that is mechanically coupled to the valve element. Compressed gas is forced into and out of the cylinder to move the connecting rod, which is mechanically coupled to the stem of the control valve. In a single-acting actuator, the compressed gas is taken in and exhausted from one end of the cylinder and is opposed by a range spring, while in a double-acting actuator, air is taken in one end of the cylinder while simultaneously exhausting it out of the opposing end.
Precise and accurate control of the valve actuator, and hence the valve element, can be achieved with a positioner device coupled thereto. Pneumatic valve positioners, which can cooperate with aforementioned pneumatic actuators, are well known in the art. The proportional movement of the actuator is accomplished by the movement of compressed gas into and out of the actuator piston. More particularly, valve positioners incorporate a spool (or other devices) that either rotates or slides axially in a housing to port the flow of compressed gas to the actuator or to one or more exhaust ports.
An electrical control circuit provides a variable current signal to the positioner device that proportionally corresponds to particular states of the actuator and hence a particular position of the control valve. The electrical control circuit and the electrical current signals generated thereby may be part of a broader process managed by a distributed control system (DCS). Generally, the electrical current varies between 4 milliamps (mA) and 20 mA according to industry-wide standards; at 4 mA the valve positioner may fully open the valve element, while at 20 mA the valve positioner may fully close the valve element. The positioner compares the received electrical signal to the current position of the actuator, and if there is a difference, the actuator is moved accordingly until the correct position is reached.
One previous solution involves an external component that monitors the variable current signal for the electrical control circuit and the actual position of the valve element, and responds by driving a lower power solenoid valve coupled to the valve actuator. These additional external components tend to be costly, and require a safe external power source with associated connectivity components. Along these lines, additional wiring and a separate junction box is required. In general, there are additional complications and costs, particularly for deployment in hazardous environments.
Another previous solution utilizes a valve positioner having normally closed on/off valves. However, the flow coefficient (Cv) of such valves is low, and oftentimes necessitate boosters for meeting stroking time requirements of most typical deployments. Furthermore, such boosters are also understood to exhibit some degree of leakage, largely negating the advantages of normally closed valves. In any case, the position of the valve actuator must be restored.
Accordingly, there is a need in the art for an improved valve positioner having zero bleed in a steady state position, and having such capability regardless of the flow coefficient of the positioner. It would be desirable for the valve positioner to be powered by the electrical current signal loop and not an external source, and therefore intrinsically safe.